Transport and infrared photoresponse properties of InN nanorods/Si heterojunction
© Kumar et al; licensee Springer. 2011
Received: 20 April 2011
Accepted: 28 November 2011
Published: 28 November 2011
The present work explores the electrical transport and infrared (IR) photoresponse properties of InN nanorods (NRs)/n-Si heterojunction grown by plasma-assisted molecular beam epitaxy. Single-crystalline wurtzite structure of InN NRs is verified by the X-ray diffraction and transmission electron microscopy. Raman measurements show that these wurtzite InN NRs have sharp peaks E 2(high) at 490.2 cm-1 and A 1(LO) at 591 cm-1. The current transport mechanism of the NRs is limited by three types of mechanisms depending on applied bias voltages. The electrical transport properties of the device were studied in the range of 80 to 450 K. The faster rise and decay time indicate that the InN NRs/n-Si heterojunction is highly sensitive to IR light.
Semiconducting group-III nitrides have attracted a lot of attention in recent years because of, mainly, the large band gap (0.7 to 6.2 eV) that can be covered by the nitrides and their alloys. Their optical properties are highly suitable for novel optoelectronic and photonic applications. Compared to all other group-III nitrides, InN possesses the lowest effective mass, the highest mobility, narrow band gap E g of 0.7 to 0.9 eV, and the highest saturation velocity [1, 2]. These properties make it an attractive material for applications in solar cells and in terahertz emitters and detectors [3–5]. InN/Si tandem cells have been proposed for high-efficiency solar cells . InN nanostructures can also be used as sensor materials for various gases and liquids . Good-quality InN layers are difficult to grow because of the low dissociation energy of InN and the lack of an appropriate substrates [8, 9]. The above constraints lead to the formation of dislocations and strain in the grown epitaxial layers resulting in the degradation of the device performance. Grandal et al.  reported that defect- and strain-free InN nanostructures of very high crystal quality can be grown by molecular beam epitaxy on silicon substrates.
Due to the distinctive properties and potential applications of nanostructures, various kinds of InN nanostructures have been grown such as nanowires (NWs), nanotubes, and nanorods (NRs) by plasma-assisted molecular beam epitaxy (PAMBE) and metalorganic vapor phase epitaxy [11, 12]. There are several reports on the growth of InN NWs or NRs on Si substrates [13, 14] and few reports on electrical transport [15, 16] but no report on infrared (IR) on/off characteristics of InN nanorods/Si heterojunction. Since silicon is a low-cost and the most sought semiconductor material, it is very important to understand the temperature-dependent transport and IR photoresponse mechanism of InN NRs/Si heterostructure prior to their adoption in the fabrication of optoelectronic device. In the present study, catalyst-free InN NRs were grown on Si substrates by PAMBE and studied the temperature-dependent transport and IR photoresponse mechanism of InN NRs/Si heterostructures.
The InN NRs were grown on n-Si (1 1 1) substrates by PAMBE system. The substrates were chemically cleaned followed by dipping in 5% HF to remove the surface oxide and thermally cleaned at 900°C for an hour in ultra-high vacuum. The substrates were exposed to the Indium (In) molecular beam at 350°C for 60 s (approximately two monolayers). Further, the substrate temperature was increased to 500°C to fabricate the NRs. The duration of NR growth was kept for 2 h. The general set of growth conditions includes indium beam equivalent pressure, nitrogen flow rate, and rf-plasma power, which were kept at 4.6 × 10-8 mbar, 1 SCCM, and 400 W, respectively. The morphological and structural evaluation of the as-grown NRs was carried out by the field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Further, the crystalline quality and lattice structure of the InN NRs were characterized by micro-Raman spectroscopy using a 514-nm line of the Ar+ ion laser at room temperature. The aluminum circular contacts of diameter 400 μm were fabricated by thermal evaporation using a physical mask. The adequate ohmic nature of the contacts to InN and Si was verified. The device transport characteristics were studied at various temperatures using the probe station attached with the KEITHLEY-236 source measure unit (Bell Electronics, Kent, WA, USA), and IR photoresponse characteristics were studied under IR source with 1, 500-nm-long pass filter.
Results and discussion
From Figure 7b, it can be seen that the barrier height (ϕ b) and the ideality factor (η) are dependent on the temperature and are attributed to the inhomogeneity at the interface . The large observed ideality factor suggests the presence of surface or interface states, indicating that the junction is far from being ideal [25, 26]. The large mismatch in the lattice parameters of InN and Si shows that there is generally a high density of interfacial states between two materials. The lattice mismatch produces a dislocation field at the junction interface that can attract a space charge and/or act as a recombination center, resulting in large ideality factor. Breitenstein et al.  introduced a model to describe ideality factors n > 2, which is based on coupled defects and donor acceptor pair recombination, both giving rise to an increased recombination current. It is stated that for a high density of defect states, hopping conduction in the defect volume may govern the reverse conductivity of the devices.
InN NRs/n-Si heterojunction was grown by PAMBE. Single-crystalline wurtzite structure of InN NRs is verified by the X-ray diffraction and HRTEM. Raman spectrum reveals two clear peaks, which correspond to the E 2(high) and A 1(LO) modes of wurtzite InN, respectively. The current transport mechanism of the NRs/Si heterojunctions were limited by three types of mechanisms depending on applied bias voltages. The observed higher value of ideality factor is probably due to the presence of defect states in InN NRs. The rapid rise and decay of infrared on/off characteristics of InN nanorods/Si heterojunction indicate that the device is highly sensitive to the IR light. The InN NRs/Si heterojunction device can be used for IR detectors.
The authors thank the Institute Nanoscience Initiative, IISc for providing the transmission electronic microscopy characterizations.
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